1. Field of the Invention
The invention relates to a method for detection of print marks by evaluation of a cyclical sensor signal from at least one contrast sensor which senses the area of printed material containing the print mark which is passed below the contrast sensor. The invention further relates to a device for detection of print marks with a sensor arrangement and a signal conditioning unit, wherein the sensor arrangement has at least one contrast sensor which for generation of a cyclical sensor signal is disposed above the area of printed material containing the print mark which is passed below the contrast sensor.
2. Discussion of Background Information
Print marks are used not only in the printing industry, but also in many other areas of the processing industry, for instance in order to monitor the alignment of packaging material in packaging machines and to detect and to compensate for any deviations. In the case of printing machines, for instance rotary or offset printing machines for sheet-fed or web printing, print marks are usually used in order to coordinate the different printing inks to be applied exactly with one another. Slight deviations of the print mark positions are compensated for automatically by the respective printing machine during the application of the next printing ink. Print marks are detected by so-called contrast sensors. With the aid of a light-sensitive component these contrast sensors generate an analogue signal, the strength of which corresponds to the color gradient of the print mark. With the aid of a programmable switching threshold an output signal is generated from which the start or end time of the print mark can be derived. The position or the width of the print mark can be calculated by means of the determined times and the current relative speed between the contrast sensor and the print mark. These values are provided to the processing or printing machines for position alignment.
By special shapes of the print mark, for example as wedge or double-wedge print marks, it is possible to detect not only the offset in the feed direction, but also the lateral offset of the printed material by evaluation of one single sensor signal from a contrast sensor.
In order to be able to evaluate the sensor signal from a contrast sensor more securely, it is necessary to calibrate this sensor or the unit which evaluates the sensor signal thereof for generation of the print mark signal, wherein in particular the switching threshold must be adapted to the respective environmental conditions, the contrast and the quality of the print mark and to the respective contrast sensor used in order to be able to detect the print mark clearly. In addition, when the printing machine, packaging machine or other machine is started up a parameterization has to be carried out initially. In this case a measuring window is usually defined in the cyclical signal pattern in which the print mark is to be detected. An incorrect calibration and/or an unsuitable parameterization can give rise to considerable costs if the errors are recognized too late.
Understandably, the problems which arise in the detection of print marks and in the evaluation of the sensor signals increase as the feeding speed of the printed material increases.